Contrast constrained aerothermal radiation correction method

ABSTRACT

Disclosed in the present invention is a contrast constrained aerothermal radiation correction method. By analyzing features of images at different intensities of aerothermal radiation, it has been discovered that the stronger the aerothermal radiation effect is, the smaller the image contrast becomes, and when thermal radiation correction is performed using a gradient fitting algorithm, it has been discovered that time consumption thereof grows exponentially with an increase in a degree of a fitting surface and with an increase in an image size. The present invention can rapidly and effectively restore an aerothermal radiation image, remarkably improving a signal to noise ratio and quality of the image.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Stage Appl. filed under 35 USC 371 of International Patent Application No. PCT/CN2016/079129 with an international filing date of Apr. 13, 2016, designating the United States, and further claims foreign priority benefits to Chinese Patent Application No. 201510988503.9 filed Dec. 24, 2015. Inquiries from the public to applicants or assignees concerning this document or the related applications should be directed to: Matthias Scholl P. C., Attn.: Dr. Matthias Scholl Esq., 245 First Street, 18th Floor, Cambridge, Mass. 02142.

TECHNICAL FIELD

The present disclosure relates to the field of an interdisciplinary science combining aero-optics, information processing and aerospace technology, and more particularly to a contrast-constrained aerothermal radiation correction method which can be applied to image preprocessing for remote sensing, detection, navigation and guidance of high-speed aircraft.

BACKGROUND OF THE INVENTION

Aero-optics is an interdisciplinary science combining aerodynamics and optics. For high-speed aircraft flying in the atmosphere, severe aero-optical effect will occur, which affects the imaging quality of an optical imaging detection system. Therefore, the aero-optical effect, along with its correction method, is an important research direction, and one of the main technical problems that restrict the development and application of high-speed aircrafts.

For high-speed aircraft with an optical imaging detection system flying in the atmosphere, a complex airflow field is formed by the interaction between an optical window and the airflow. Due to air viscosity, the airflow in contact with the surface of the optical window will be retarded, so that the airflow velocity decreases and a boundary layer is formed near the surface of the optical window. Within the boundary layer, the airflow layers with a large velocity gradient will have strong friction, converting kinetic energy of the airflow irreversibly into heat, and leading to rise of the temperature in the wall of the optical window. The high-temperature airflow will continuously transfer heat to the low-temperature walls, causing strong aerothermal heating. The optical window is aerothermal-heated and hence in a severe aerothermal environment; as a result, it produces thermal radiation noise, reduces signal-to-noise ratio and degrades image quality of the optoelectronic detection system.

The greater the flight speed, the more severe the aerothermal heating on the surface of the aircraft. The irradiance of the airflow outside of the optical window and the irradiance of the optical window are superimposed on the irradiance of background; as a result, an imaging sensor will enter a non-linear operation range or saturate, causing loss of effective information of scenes and reduction of signal-to-noise ratio and signal-to-clutter ratio, thus degrading the detection performance. Therefore, it is necessary to perform aerothermal radiation correction on the images acquired by the imaging sensor, in order to improve the image quality. As degradation model of aerothermal radiation is unknown and randomly changed and the degraded images contain other noise, these increase the difficulty of image restoration or correction. In addition, for specific applications in high-speed aircrafts, especially hypersonic aircrafts, the high-frame-rate characteristic of their imaging systems demands that a correction algorithm have extremely high real-time performance.

SUMMARY OF THE INVENTION

In view of the above-described problems, it is one objective of the invention to provide a contrast-constrained aerothermal radiation correction method configured to solve the defects of the conventional aerothermal radiation correction methods, i.e., poor results in handling strong thermal radiation effect and low correction efficiency in processing large-size images; the method provided by the present disclosure can be used for aerothermal radiation correction on images acquired during remote sensing, detection, navigation and guidance of a high-speed aircraft.

To achieve the above objective, in accordance with one embodiment of the invention, there is provided an aerothermal radiation correction method, the method comprising:

-   -   (1) filtering out noise and details in an original aerothermal         radiation image Z, thus obtaining a filtered image {circumflex         over (Z)}, to overcome the adverse effects of noise in thermal         radiation field estimation process;     -   (2) through estimation of the filtered image {circumflex over         (Z)}, obtaining an aerothermal radiation field B₁ of the         original aerothermal radiation image Z, and further obtaining an         initially corrected image S₁=Z−B₁;     -   (3) solving a central region of the aerothermal radiation field         B₁, and according to the central region of the aerothermal         radiation field B₁, dividing the original aerothermal radiation         image Z and the initially corrected image S₁ into         correspondingly equal-sized image-blocks;     -   (4) calculating the contrast values of the image-blocks of the         original aerothermal radiation image Z and the contrast values         of the image-blocks of the initially corrected image S₁,         respectively, thus obtaining the variation of the contrast         values of the image-blocks of the original aerothermal radiation         image Z relative to the corresponding image-blocks of the         initially corrected image S₁;     -   (5) comparing the variation of the contrast values of the         image-blocks corresponding to the central region of radiation         and the variation of the contrast values of the image-blocks         corresponding to non-central regions of radiation, if the         difference is less than or equal to a predetermined threshold         value, then taking the initially corrected image S₁ as the final         correction result, otherwise sequentially proceeding to step         (6); and     -   (6) obtaining an image         ₁={circumflex over (Z)}−B₁ from the filtered image {circumflex         over (Z)} and the aerothermal radiation field B₁, and taking the         portion of the image         ₁ corresponding to the radiation central region of the         aerothermal radiation field B₁ as a new filtered image         {circumflex over (Z)}, and through estimation of the new         filtered image {circumflex over (Z)}, obtaining a residual         aerothermal radiation field B₂ at the radiation central region         of the initially corrected image S₁, and further obtaining a         secondarily corrected image S₂=S₁−B₂.

In a class of this embodiment, the above method further comprises a step (7) as follows: performing weighted processing to the edges of the radiation central region of the secondarily corrected image S₂ and the edges of the radiation central region of the initially corrected image S₁, to eliminate the edge effect caused by block division, to achieve higher quality of the images.

In a class of this embodiment, in step (3), taking the radiation central region of the aerothermal radiation field B₁ as the center, and dividing the original aerothermal radiation image Z and the initially corrected image S₁ into correspondingly equal-sized image-blocks, in such a way that the image-blocks corresponding to the radiation center regions of the aerothermal radiation field B₁ and of the original aerothermal radiation image Z are located at the center of all the image-blocks of the original aerothermal radiation image Z, and that the image-blocks corresponding to the radiation center regions of the aerothermal radiation field B₁ and of the initially corrected image S₁ are located at the center of all the image-blocks of the initially corrected image S₁.

In general, the above technical solution contemplated by the present disclosure has the following advantageous effects as compared with the prior art: by analyzing features of aerothermal radiation images with different intensities, it is found that, for an image, the stronger the aerothermal radiation effect, the smaller the contrast. When thermal radiation correction is carried out by using a gradient fitting algorithm, it is found that, the time consumption increases exponentially with the increase of the order of the fitted curved-surface and the image size; the present disclosure can achieve quick and effective aerothermal radiation image restoration, thereby significantly improving signal-to-noise ratio and quality of images.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing image contrast value varies with intensity of aerothermal radiation, by adding a curved-surface with gradually-increased irradiance to an input image and calculating variance of contrast values;

FIG. 2 is a graph showing time consumption varies with fitting order K, in a correction method of gradient least-squares fitting;

FIG. 3 is a graph showing time consumption varies with image size, in a correction method of gradient least-squares fitting;

FIG. 4 is a flowchart of the contrast-constrained aerothermal radiation correction method according to an embodiment of the present disclosure;

FIGS. 5A and 5B show the filtering effect of weighted least-squares (WLS), where, FIG. 5A is an original image, FIG. 5B is a filtered image;

FIGS. 6A, 6B, 6C, 6D, 6E, 6F, 6G, and 6H show an example of the contrast-constrained aerothermal radiation correction method, where, FIG. 6A is a reference image; FIG. 6B is an aerothermal-radiation degraded image; FIG. 6C is the result of WLS filtering of FIG. 6B; FIG. 6D is the result of initial correction of FIG. 6B, through gradient fitting of an aerothermal radiation curved-surface, in the case of K=2; FIG. 6E is a schematic diagram illustrating the image-blocks division to the fitted aerothermal radiation field of FIG. 6D in the case of K=2 and the initially corrected result; FIG. 6F is the result of secondary correction of the central region of FIG. 6E; FIG. 6G is the result of edge fusion of FIG. 6E; FIG. 6H is a graph of a column of pixels taken from FIG. 6G, for verification of the result and effect of aerothermal radiation correction;

FIGS. 7A, 7B, and 7C show comparison of the contrast-constrained aerothermal radiation correction method and a unconstrained aerothermal radiation correction method, where, FIG. 7A is the correction result in the case of K=9; FIG. 7B is the result of contrast-constrained correction; FIG. 7C is graph of a column of pixels taken respectively from a reference image, an unconstrained-corrected image, and a contrast-constrained-corrected image, showing the respective effects; and

FIGS. 8A, 8B, 8C, and 8D show the respective effects of electric-arc-wind-tunnel experiments, where, FIG. 8A is an electric-arc-wind-tunnel aerothermal radiation image; FIG. 8B is the result of an unconstrained correction; FIG. 8C is the result of contrast-constrained correction; FIG. 8D is a graph of a column of pixels taken from the respective results, showing the respective effects.

DETAILED DESCRIPTION OF THE EMBODIMENTS

For further illustrating the invention, experiments detailing a contrast-constrained aerothermal radiation correction method are described hereinbelow combined with the drawings. It should be noted that the following examples are intended to describe but not to limit the invention.

The present disclosure is based on the following three characteristics of thermal radiation correction: (1) image contrast value decreases with gradual increase of aerothermal radiation intensity, as shown in FIG. 1; (2) when thermal radiation correction is carried out by using gradient fitting, the time consumption increases exponentially with increase of fitting order K, as shown in FIG. 2; (3) when thermal radiation correction is carried out by using gradient fitting, the correction time consumption increases approximately in exponential form with increase of image size, as shown in FIG. 3; therefore, a contrast-constrained aerothermal radiation correction method is proposed.

As shown in FIG. 4, the contrast-constrained aerothermal radiation correction method according to an embodiment of the present disclosure comprises the following steps:

(1) Due to the low-frequency characteristics of an aerothermal radiation field, using an image-smoothing algorithm based on WLS to filter out noise and details in an original aerothermal radiation image Z, thus obtaining a filtered image {circumflex over (Z)}, to overcome the adverse effects of noise in thermal radiation field estimation process;

Specifically, the filtered image {circumflex over (Z)} is expressed as: {circumflex over (Z)}=(I+λL)⁻¹ Z, where, I is a unit matrix; λ is a smooth parameter, the larger λ, the smoother the filtered image, and in this embodiment, it is set to 0.002; L=D_(x) ^(T)W_(x)D_(x)+D_(y) ^(T)W_(y)D_(y), D_(x) and D_(y) are forward difference operators in x direction and y direction of the image coordinate system, respectively; W_(x) and W_(y) are weighting-factor diagonal matrixes in x direction and y direction of the image coordinate system, respectively.

Specifically, the value of the diagonal element in the i-th row of W_(x) is

$\left( {{{\frac{\partial l}{\partial x}(i)}}^{\alpha} + ɛ} \right)^{- 1},$ the value of the diagonal element in the i-th row of W_(y) is

$\left( {{{\frac{\partial l}{\partial y}(i)}}^{\alpha} + ɛ} \right)^{- 1},$ where, l is the logarithmic transformation of an input image Z;

$\frac{\partial l}{\partial x}$ and

$\frac{\partial l}{\partial y}$ represent gradients in x direction and y direction, respectively; α is a constant for control of a filter's sensitivity to the gradient of an original image, the larger α, the more sensitive to the gradient of the original image, and the stronger edge-retention of the image after filtering, and in this embodiment, it is set to 1.1; ε is a relatively small constant for preventing denominator from being 0, and in this embodiment, it is set to 0.00001.

FIGS. 5A and 5B show the WLS filtering effect, where, FIG. 5A is an original image, FIG. 5B is a filtered image; it is apparent from the figure, after the WLS filtering processing, most of the details of the image are filtered out.

Based on this feature of WLS, the filtering process is performed on aerothermal-radiation degraded images; FIG. 6A is an original infrared image, FIG. 6B is an aerothermal-radiation degraded image; FIG. 6C is the image after the WLS filtering processing, from which high-frequency components (noise and details) are filtered out, making estimation of a low-frequency aerothermal radiation field in subsequent steps more accurate.

(2) Based on the feature that an aerothermal radiation field B can be fitted by a K-order two-dimensional polynomial, using a least-squares method for estimation of the filtered image {circumflex over (Z)}, thus obtaining the aerothermal radiation field B of the original aerothermal radiation image Z; in this step, the aerothermal radiation field obtained through estimation with the setting K=2 is denoted as B₁, and in the subsequent step (6), the initially corrected residual aerothermal radiation field obtained through estimation is denoted as B₂.

Further, from the obtained aerothermal radiation field B₁, an initial corrected image S₁=Z−B₁ is obtained;

${B\left( {x,y} \right)} = {{\sum\limits_{t = 0}^{K}{\sum\limits_{s = 0}^{K - t}{a_{t,s}x^{t}y^{s}}}} = {Ca}}$ where, (x, y) are coordinates of a pixel; a is polynomial coefficient; C is a constant matrix for substituting x and y into the above equation; K is the order of the two-dimensional polynomial.

arg  min ∇Ẑ − ∇B₂²

A least-squares method is employed for estimation, and when the L2 norm of the difference between the gradient ∇{circumflex over (Z)} of the filtered image {circumflex over (Z)} and the gradient ∇B of the aerothermal radiation field to be estimated arrives at a minimum solution, that is the solved aerothermal radiation field B, as shown by the above formula.

In the solving process of this approach, the time consumption has the trend of exponential growth with the increase of K, as shown in FIG. 2; therefore, in the first use of the above approach, the aerothermal radiation field B₁ is solved with the setting K=2, thus obtaining the initial corrected image S₁, as shown in FIG. 6D, and by subtracting the radiation field B₁ from the filtered image, an image

₁ is obtained,

₁ is used as a filtered image and as an input image in subsequent steps for secondary estimation of radiation field. It can be seen from the initially corrected image that, most of the radiation field of the image has been filtered out, and only the radiation central region remains relatively strong radiation field; therefore, the subsequent steps focus on estimation and removal of the thermal radiation field at the radiation central region.

(3) Solving the radiation central region of the aerothermal radiation field B₁, wherein the radiation central region corresponds to the region having relatively large gray-scale values in the thermal radiation image; and according to the radiation central region of the aerothermal radiation field B₁, dividing the original aerothermal radiation image Z and the initially corrected image S₁ into correspondingly equal-sized image-blocks;

For example, by utilizing the solved radiation central region, block-division processing is performed on the input aerothermal radiation image (as shown in FIG. 6B) and the initially corrected image (as shown in FIG. 6D), thus, with the radiation central region as the center, the images are each divided into 9 image-blocks, and the result is as shown in FIG. 6E.

(4) Calculating the contrast values of the image-blocks of the original aerothermal radiation image Z and the contrast values of the image-blocks of the initially corrected image S₁, respectively, thus obtaining the variation of the contrast values of the image-blocks of the original aerothermal radiation image Z relative to the corresponding image-blocks of the initially corrected image S₁;

For example, a method for calculating the contrast value of an image-block is as follows:

Calculating the sum of the squares of the difference between the gray-scale value of each pixel and the gray-scale values of the four adjacent pixels, then dividing the sum by the total pixel number of the image block. Ctr=Σ _(δ)δ(i,j)² P _(δ)(i,j) where, δ(i, j) is the gray-scale difference between adjacent pixels; P_(δ)(i, j) is the distribution probability of a pixel with gray-scale difference between adjacent pixels being δ; Ctr is the contrast value of the image.

With the above contrast-value calculation formula, the contrast values of the image-blocks of the original aerothermal radiation image Z and the contrast values of the image-blocks of the initially corrected image S₁ are calculated respectively, as shown in Table 1 and Table 2. Table 1 shows the contrast values of the respective image-blocks of the original aerothermal radiation image Z, Table 2 shows the contrast values of the respective image-blocks of the initially corrected image S₁, and Table 3 shows the difference between the contrast values of the corresponding image-blocks of both the original aerothermal radiation image Z and the initially corrected image S₁.

TABLE 1 41.5 26.7 83.2 43.7 18.0 70.3 100.3 37.1 22.8

TABLE 2 126.0 169.8 337.4 220.9 44.6 271.5 237.3 126.2 103.8

TABLE 3 84.4 143.1 254.2 177.1 26.5 201.1 137.0 89.1 80.9

(5) Based on the feature that, for an aerothermal radiation image, the stronger the radiation, the smaller the contrast, comparing and analyzing the variation of the contrast values of the image-blocks of the original aerothermal radiation image Z relative to the corresponding image-blocks of the initially corrected image S₁, which is obtained in step (4); if the variation of the contrast values at the central region of radiation is significantly less than the variation of the contrast values at non-central regions of radiation, that is, the difference is greater than a predetermined threshold value T, which indicates that: the radiation intensity of the original aerothermal radiation image Z is quite high, although the effect of correction for non-central regions of radiation has reached to a relatively high level, the central region of radiation still remains relatively strong aerothermal radiation noise, hence, it is necessary to perform secondary correction to the central region of radiation, thus sequentially proceeding to step (6);

Contrarily, if the variation of the contrast values at the central region of radiation is slightly different from the variation of the contrast values at non-central regions of radiation, that is, the difference is less than or equal to a predetermined threshold value T, which indicates that: the radiation intensity of the original aerothermal radiation image Z is relatively low, so the image has been well corrected, thus taking the initially corrected image S₁ as the final correction result;

It can be clearly seen from Table 3 that, the restoration of the contrast value at the central region of radiation is significantly lower than that at the peripheral regions, which indicates that the central region of radiation still remains relatively strong aerothermal radiation noise, so it is necessary to perform secondary correction to the central region of radiation.

(6) Obtaining an image

₁={circumflex over (Z)}−B₁ from the filtered image {circumflex over (Z)} resulted in step (1) and the aerothermal radiation field B₁ resulted in step (2), and taking the portion of the image

₁ corresponding to the radiation central region of the aerothermal radiation field B₁ as a new filtered image {circumflex over (Z)}, and through estimation of the new filtered image {circumflex over (Z)}, obtaining a residual aerothermal radiation field B₂ at the radiation central region of the initially corrected image S₁, and further obtaining a secondarily corrected image S₂=S₁−B₂;

In the fitting process herein, since only the relatively small region at the center of the image is subjected to the fitting process, it is possible to use a higher order K—compared with the case in step (2)—for the fitting process, without causing a high cost of time consumption; in this embodiment, K=5. By subtracting the estimated aerothermal radiation field at the central region from the central region of the initially corrected image, a secondary corrected image S₂ is obtained, as shown in FIG. 6F.

(7) Due to the edge effect inevitably brought by block division processing of images, in this step, performing weighted processing to the edges of the radiation central region of the secondarily corrected image S₂ and the edges of the radiation central region of the initially corrected image S₁, to eliminate the edge effect caused by block division, to achieve higher quality of the images.

It can be seen from FIG. 6F that, the aerothermal radiation noise has been completely removed, but the edge effect brought by block division still remains; the image after weighted processing is as shown in FIG. 6G. FIG. 6H shows a graph of a column of pixels taken from the original image, the aerothermal radiation image and the contrast-constrainted corrected image, respectively, and it can be seen from the graph that, after processing according to the method of the present disclosure, the aerothermal radiation effect in the image has been well corrected.

FIGS. 7A, 7B, and 7C show the result of aerothermal-radiation-field gradient fitting by directly using K=9 (FIG. 7A) and the result of contrast-constrained secondary correction (FIG. 7B); it is apparent from the figure that, directly using high-order fitting leads to generation of a relatively strong halation ring at the central region of aerothermal radiation, thus affecting image quality, whereas contrast-constrained secondary correction overcomes this problem; FIG. 7C is a graph of a column of pixels taken from an original image, an aerothermal radiation image, a corrected image by using high-order (K=9) two-dimensional polynomial fitting and a contrast-constrained-corrected image, respectively, showing that the method of the present disclosure reduces correction time consumption while guaranteeing correction effect.

Below, Table 4 shows comparison of the respective parameters, such as time consumption and PSNR (peak-signal-to-noise ratio), of a direct correction method and the contrast-constrained correction method in correction of the same aerothermal radiation image, and it can be clearly seen from the table that, the contrast-constrained correction method greatly reduces correction time consumption, with slightly increase of PSNR.

TABLE 4 Images T(s) PSNR(dB) Aerothermal radiation — 12.34 image Directly corrected 0.7393 19.23 image Contrast-constrained- 0.4322 22.85 corrected image

FIGS. 8A, 8B, 8C, and 8D are an example for correcting an electric-arc-wind-tunnel image, where, FIG. 8A is a wind-tunnel aerothermal radiation image, FIG. 8B is the corrected image by using high-order (K=9) two-dimensional polynomial fitting, FIG. 8C is the contrast-constrained-corrected image, FIG. 8D is a graph of a column of pixels taken from the electric-arc-wind-tunnel aerothermal radiation image, the corrected image by using high-order (K=9) two-dimensional polynomial fitting and the contrast-constrained-corrected image, respectively.

Unless otherwise indicated, the numerical ranges involved in the invention include the end values. While particular embodiments of the invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention. 

The invention claimed is:
 1. An aerothermal radiation correction method, the method comprising: (1) filtering out noise and details in an original aerothermal radiation image Z, thus obtaining a filtered image {circumflex over (Z)}, to overcome the adverse effects of noise in thermal radiation field estimation process; (2) through estimation of the filtered image {circumflex over (Z)}, obtaining an aerothermal radiation field B₁ of the original aerothermal radiation image Z, and further obtaining an initially corrected image S₁=Z−B₁; (3) solving a central region of the aerothermal radiation field B₁, and according to the central region of the aerothermal radiation field B₁, dividing the original aerothermal radiation image Z and the initially corrected image S₁ into correspondingly equal-sized image-blocks; (4) calculating the contrast values of the image-blocks of the original aerothermal radiation image Z and the contrast values of the image-blocks of the initially corrected image S₁, respectively, thus obtaining the variation of the contrast values of the image-blocks of the original aerothermal radiation image Z relative to the corresponding image-blocks of the initially corrected image S₁; (5) comparing the variation of the contrast values of the image-blocks corresponding to the central region of radiation and the variation of the contrast values of the image-blocks corresponding to non-central regions of radiation, if the difference is less than or equal to a predetermined threshold value, then taking the initially corrected image S₁ as the final correction result, otherwise sequentially proceeding to step (6); and (6) obtaining an image

₁={circumflex over (Z)}−B₁ from the filtered image {circumflex over (Z)} and the aerothermal radiation field B₁, and taking the portion of the image

₁ corresponding to the radiation central region of the aerothermal radiation field B₁ as a new filtered image {circumflex over (Z)}, and through estimation of the new filtered image {circumflex over (Z)}, obtaining a residual aerothermal radiation field B₂ at the radiation central region of the initially corrected image S₁, and further obtaining a secondarily corrected image S₂=S₁−B₂.
 2. The method of claim 1, further comprising a step (7) as follows: performing weighted processing to the edges of the radiation central region of the secondarily corrected image S₂ and the edges of the radiation central region of the initially corrected image S₁, to eliminate the edge effect caused by block division, to achieve higher quality of the images.
 3. The method of claim 1 or 2, wherein in step (3), taking the radiation central region of the aerothermal radiation field B₁ as the center, and dividing the original aerothermal radiation image Z and the initially corrected image S₁ into correspondingly equal-sized image-blocks, in such a way that the image-blocks corresponding to the radiation center regions of the aerothermal radiation field B₁ and of the original aerothermal radiation image Z are located at the center of all the image-blocks of the original aerothermal radiation image Z, and that the image-blocks corresponding to the radiation center regions of the aerothermal radiation field B₁ and of the initially corrected image S₁ are located at the center of all the image-blocks of the initially corrected image S₁. 